Various convenient methods for producing prepared food items and processing food items have been proposed in recent years. For instance, a method of providing a fresh prepared food item without a preservative (Patent Literature 1) and a method of cooking meat that can prevent a change in flavor or color of the meat (Patent Literature 2) have also been proposed. However, the food processing methods as described in Patent Literatures 1-2 are unsuited for processing a large quantity of food, resulting in a low yield and increased cost.
Various methods have been proposed as means for preventing an increase in cost in processing of food. For example, Patent Literature 3 describes that a continuous heating apparatus with two stages of heating means is used for reducing the frequency of exchanging cooking oil and producing fried food with excellent mouthfeel.
Patent Literature 4 describes that a high temperature hot water is contacted with food to improve the sterilization level of the food and to shorten the time of cooking such as boiling, simmering, or steaming.
Patent Literature 5 describes that an acid, alkali, or alcohol is used to preprocess food and then the food is heated under moderate conditions to produce a cooked food item with excellent flavor and preservability.
Patent Literature 6 describes that food is simultaneously sterilized and cooked in a short period of time by repeatedly reducing pressure and heating when cooking the food in a sealed tank.
Furthermore, extended cooking time and complex cooking methods are avoided, and many consumers believe that nutrients or mouthfeel of food is lost in food items heated at a high temperature, such as canned and retort package food items. Thus, home electronic products that can shorten cooking time (Patent Literature 7) and a method of using steam for heating and cooking under relatively moderate conditions (Patent Literature 8) have been proposed.
Especially in recent years, processed products of fresh vegetables/fresh fruits such as cut vegetables and cut fruits are becoming more prevalent. Cut vegetables are products consisting of finely cut vegetables that are filled in a container such as a bag or a plastic cup. Consumers can make salad simply by transferring the content from the container to tableware. Cut vegetables include not only products for salad, but also products for stir-frying. Consumers can directly transfer the content from a container to a pot or a frying pan for heating. By using cut vegetables, consumers are relieved of hard labor involving purchasing and carrying heavy or bulky vegetables and cutting the vegetables into a suitable size for the dish. Patent Literatures 9-13 describe a method of producing such a convenient product, i.e., cut vegetables.
However, even now, with the prevalence of low temperature transport and low temperature shelves, cut vegetables and cut fruits face problems, such as surface discoloration of fruits and vegetables and the tendency thereof to readily spoil. For this reason, discoloration prevention or sterilization is considered essential for the production of cut vegetables and cut fruits (Patent Literatures 11, 12, and 13).
For example, Patent Literature 14 describes a method of sterilizing and washing fresh food items using an aqueous solution of water-soluble chlorine dioxide and an emulsifier. This method can prevent corrosion of an apparatus or oxidation of food items with a sterilizing agent. However, this method requires an emulsifier in addition to a sterilizing agent, leading to increased cost. Moreover, a sterilizing effect is only confirmed up to 48 hours after treatment.
Patent Literature 15 describes a method for efficient sterilization by having fresh food items contact with or immersed in an aqueous mixture solution of a chlorine-based sterilizing agent, polyoxyethylene sorbitan fatty acid ester and an organic acid and/or a salt thereof. However, this method also requires the use of multiple additional agents for the sterilization. Moreover, there is a risk of impairing the flavor of food by thorough washing and draining that are required after sterilization. In this manner, it is difficult to omit all of chemical agents, washing with water, and draining in a sterilization method using a chlorine-based sterilizing agent, such that there are remaining problems in terms of cost and quality of food.
As means for solving such a problem involving the use of a chlorine-based sterilizing agent, Patent Literature 16 describes a quality retaining agent for food items containing glycerol and monoglycerol fatty acid ester. Drying or discoloration of food items can be suppressed even 10 days after processing of food with a quality retaining agent. However, this is the same as conventional sterilization methods in terms of using a certain type of agent in order to maintain quality. Moreover, the effect of quality retaining agents on the flavor of food items and the effect thereof on the human body when ingested in a large quantity have not been sufficiently studied. Further, a method of washing vegetables to aseptic levels using ozone or ultrasound has been proposed. However, the cost of a washing facility would be high, so that a conversion from a current method using a sterilizing agent is not simple.